Electronic signal mixer



Dec. 2, 1952 R. D. MdcoY ErAL 2,620,441

' ELECTRONIC SIGNAL MIXER Filed o'ct. 24, 194e 2 SHEETS-SHEET 1 FILTER CRSE RHWLEY 0. M 7//000505 d. h

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Dec. 2, 1952 R. D. MocoY ET AL ELECTRONIC SIGNAL. MIXER Filed Oct. 24, 1946 (gaan/m5739191 70/1 INVENTORS RA wLsY D. Mc COV 77/00051/5 d. MUS T0 Patented Dec. 2, 1952 UNITED STATES PATENT OFFICE ELECTRGNIC SIGNAL MIXER Rawley D. M'cCby, Bronxville, and Thaddeus J. Kusto', Ne'w York, N. Y., assignors to The Sperry Corporation, a corporation of Delaware Application October-.'24, 1946, Serial No. 705,292

(Cl. Z50-27) 4. Claims.

Our invention relatesgenerallyfto an improved signal voltagemixing circuit adapted to provide a voltage output having' components correspondingv to a pair of signal voltage inputs. Our invention particularly relates to a signal mixing circuitrrfcr combining. ne andA coarse error signalfvolt'ages and'onefwhich is particularly. adapted for.y use. in acontrol circuit` for a -servomoton Fine and coarse errorsignalsystemsfare ordim narily employed in servomotor systems to provide a closevand quite accurate control'over the ser-vomotor sov that, for example, the load shaft or positionable object driven thereby.- will followl an input shaft or reference. member withV extremely small lag, or leadr therebetween. Ordinarily, line and coarse errorV systems,V such as those herein illustrated and hereinafter described, respectively include a signal vtransmitter and a signal receiver or transformer. The transmitter of thecoarse systemis ordinarily connected directly with the data input orwith the reference member so that the rotor of thetransmitter ro tates in a 1:1v ratio with respect thereto. The rotor of the fine transmitter, on the other hand, is-rotated in some xed, higher ratio with respect thereto, such as, for example, a 36:1 or

in any other desired ratio with respect tothe. The coarse` reference memberk or input shaft. signal errory voltagev derived'from thefcoarse systemwill therefore vary in amplitude directlywith rotation of Vtheinput shaft, or, moreprecisely the amplitude of `the voltage envelope of the output voltage of the coarse system Will vary in amplitude directly with rotation oi the input shaft. The fine error voltage or thevoltage envelope of the outputfvoltage of the fine system will vary in amplitudein the adopted vratio as, for example, 36:1. with respect to rotation ofthe input shaft. These signal. voltages are ordinarily supplied. to

an amplifier which'iisassociated in controlling.

relationship with the servomotor or other controlld device.

Since fineand coarse error voltages are employed toprovide close and accurate tracking of the positionable object, driven by. the servo, with.

thefreference member, ancontrolmust be exercised cverthese signal'voltages in order that theyl may be.V fully utilizedY under. thoseA conditions where, their, presence is. mostn needed!A fr proper control, and may yet 'be modi'ed or elimi-l ourinvention as embodiedin a servomotor con-A trol system mainly forexemplary purposes and also as illustrative of` the use to which the invention is particularly adapted.

It is the primary object of this invention to provide an improved mixingv circuit for a pair of voltages such as those derived from a iine and coarse selsyn data transmission system, the voltages being combined in afpredeterm'ined manner;

Another object of this invention resides'in-prvidingan electronic mixing circuit in which the output voltage contains components corresponding to a pair of input signal voltages, the magnitude of one ofsaidvoltage components which corresponds to a rst of said signal voltages vbeing controlled by the other signal voltage.

Another object resides in providing electronic means controlled by one signal voltage for supplyinga signal voltage component in theV output thereof corresponding to said controlling voltage' and which electronic means'controls themagniV tude of a second signal voltage or a voltage com# ponent corresponding thereto in said output.V

Another object residesin providing an elec--v tronic'circuit or electronic means of thefore going characters which are so arranged as pro-l gressively to reduce the magnitude of the signal voltage component in the output corresponding to one signal voltage as the other signal voltage increases.

A still further object resides in providing a mixing circuit comprising electron tube' means controlled by one signal voltage for providing a voltage component in theV plate'circuit thereof corresponding to said one signal voltage and in wh'ich'a second signal voltage is connected with saidplate circuit whereby the output voltage will include components corresponding to both of said signal voltages, said tube means functioning to control the magnitude of both voltage components in said plate circuit.

Another object resides in providing a circuit of the character-'last above recited in which the electron tube means is `so arrangedthat no' voltapplied to the control electrode thereof will appear in the plate circuit or output for values of the voltage applied to said control electrode below a predetermined magnitude.

Another object resides in providing a circuit of the character last above set forth in which the voltage component in the output corresponding to the said second of the pair of signal voltages decreases for values of the first signal voltage above the predetermined magnitude above referred to.

The invention in another of its aspects relates to novel features of the instrumentalities described herein for achieving the principal objects of the invention and to novel principles employed in those instrumentalities, whether or not these features and principles are used for the said principal objects or in the said field.

A further object of the invention is to provide improved apparatus and instrumentalities embodying novel features and principles, adapted for use in realizing the above objects and also adapted for use in other fields.

Still another object of this invention resides in providing, in an electronic servomotor controller, an electron or thyratron tube circuit in which the bias and signal pot-entials are applied in parallel to the control electrode or grid of said tubes; and in' providing such a circuit in which the bias voltage is supplied through series condensers which permit parallel grid excitation and at the same time form a shunt for filtering out harmonics.

. With the foregoing and still other objects in view, our-invention includes the novel elements and the combinations and arrangements thereof described below and illustrated in the accompanying drawings, in which- Fig. 1 schematically represents a servomotor control system embodying the preferred form of our signal mixing circuit which is illustrated in the detailed wiring diagram;

Fig. 2 illustrates in greater detail a portion of the control system shown in Fig. 1; and

Fig. 3 shows curves representing th-e voltage components and resultant voltage in the output of our mixing circuit.

Fig. l illustrates somewhat schematically a positional control system as illustrative of systems embodying the present invention. In Fig. 1, I indicates generally the input shaft with which a reference member may be connected and to which the rotor 2 of the transmitter 3 in the coarse signal system is directly coupled or so coupled thereto that the reference member or shaft I and the rotor 2 rotate together. The input shaft I is also coupled through a gear train, indicated generally at Il, with the rotor 5 of a transmitter E which is the transmitter in the flne signal system. The gear train d is designed, as herein assumed for descriptive purposes, to drive the rotor 5 in a ratio of 36:1 with respect to the input shaft or rotor 2 of the coarse transmitter.

The stator windings of the transmitters 3 and 6 as indicated generally at 'I and 8, respectively, are connected together in polycircuit fashion and with the correspondingly disposed and similarly connected stator windings 9 and IB of the signal transformers II and I2. Selsyn or Autosyn type units may be employed as the transmitters and as the receivers or signal transformers in the above-described transmission systems, and it will be understood that where in the following description we may refer to selsyns that we mean to include any suitable type unit which may be employed for the purposes herein specied.

The rotors of the transmitters 3 and 6 are connected across a source of suitable alternating current indicated generally at I3, while the rotors I4 and I5 of the receivers or signal transformers II and I2 are connected tosupply signal voltages to the mixing and signal voltage control circuit included within the dot-dash line and indicated generally at I6. The output of the line transmission system or rotor I5 of the signal transformer I2 is connected across the input taps II and I3, while the output of the coarse transmission system or rotor Id of signal transformer II is connected across the input terminals I9 and 20.

The output of the voltage mixing circuit will be applied across the terminals 2l and 22. The two inputs comprise respectively the terminals II, I8 and I9, 2G. In the embodiment shown, the fine signal voltage which is applied across terminals I'I and I3 appears across potentiometer 23 providing a voltage adjustment. One end of potentiometer 23 is connected through resistor 22 to the plate of an electron tube 25, while the slider on the potentiometer 23 is connected through resistor 26 to the plate of a second electron tube 2l. In the preferred arrangement of our invention, these two tubes are connected in push-pull relation, the cathodes thereof being connected together and through biasing resistors 2S and 29 with a suitable source of D. C. supply 33. The plate-cathode circuits of these tubes are completed by the plate resistors 3l and 32 across which the output voltage of these tubes appears. The control electrode or grid circuits of these tubes include the common resistor 28 and, respectively, the secondary windings 33 and 34 of an input transformer 35.

It will be noted that the negative terminal of the D. C. source is connected to a common point between the resistor 28 and the transform-er secondaries 33 and 34, while the positive terminal of the D. C. supply is connected to the midpoint between output resistors 3I and 32. This serves to bias both tubes to some predetermined value below cut-off so that they will conduct only when the signal voltage supplied to the grids thereof through the transformer secondaries exceeds some predetermined value as hereinafter more fully pointed out.

The output of the rotor i4 of signal transformer II is connected in series with the secondary of transformer 36 across the input terminals I9 and 2B. Transformer 36 serves to supply an alternating voltage of substantially constant amplitude which is ordinarily termed an anti-stickoif voltage to prevent synchronization ofthe load in a relation with the reference member which is driven with the input shaft I. The possibility of synchronization in this manner occurs ,when an even numbered ratio of ne tov coarse data transmission system is used.

In the present illustration, we have assumed a 36:1 ratio and hence utilize an anti-stick-oif voltage. Hence, a combined coarse error voltage and anti-stick-oif voltage is applied across the input terminals I9 and 25 and across both primary windings 37 and 38 of input transformer 35. Y

The operation of the voltage mixing circuit above described is as follows. The two tubes 25 and 21 are biased below cut-olf and, for example, so biased that for coarse signal voltages representing less than 20 minutes of actual error or angular amount of positional disagreement between the reference `member and positionable object,rthesetubes will notconduct. However,

when the actual error represented by thecoarse signal voltage does exceed 20 minutes, the bias on these,4 tubes will be overcome and conduction will occur. Therefore, for values of coarse error below a predetermined one, such as 20 minutes, no voltage component corresponding to the coarse error voltage will appear across the output terminals of this circuit but the iine error voltage whichis connected to the plates of these tubes andV with the output terminals 2| and 22 will, be present in the output. As.. soon as the tubes 25 and 21 conduct, they will serve not only to supply an output voltage component corresponding to the coarse error voltage but valso to attenuate the fine error voltage. The time during which these tubes conduct; gradually increases as the coarse error voltage increases and hence the effective shunting of the line signal is not abrupt but also gradually increases as the` error increases above the predetermined value, such as 20 minutes. This'shunting of the fine error may progressively increase until the line error volt-` age component in the output of the circuit is reduced to zero, which may occur at a value of approximately 2 of actual error.

Fig. 3 illustrates the relative magnitude of the envelopes of the voltage components corresponding to fine and coarse signal voltages in the output of our mixing circuit plotted against actual error in degrees. 'The dotted line 39 represents the ine error voltage component, the dot-dash curve 4|) represents the coarse error voltage cornponent, and the full line curve 4| represents the summation of these components or the character ofthe control voltage employed in controlling the servomotor. Tubes 25- and 21 may, of course, amplify the coarse signal voltage.

Also, it will be noted that signal mixing occurs in such a manner that the resultant control voltage-Will not decrease materially, as curve 4| indicates, until the 180 error position is reached. However, the slope of curve-4| at or adjacent the 180 error point 4vvillbe substantially the same or as great as it is at the true reference position, thereby eliminating any possibility of synchronizing at a false point.

Referring again to Fig. 1, in the amplier or. control circuit herein shown, theoutput of the mixingcircuit is supplied through coupling condensers42 and 43 to the .grids of a second stage including electronic tubes 44 and 45. This stage is, provided mainlyso that the resultant signal voltage may workinto a high impedance circuit, and,v some ampliiication may be obtained in thisstage.

The ,outputs of tubes 44` and 45 are supplied through coupling transformer 46 to a demodulator 41 which ispreferably of a full Wave character and may include error rate and error integrating circuits. This demodulator forms no part" of the present invention but preferably includes impedance comprising resistance and capacitance connected in inverse feedback circuits to provide error rate and error integral eiects in the unidirectional voltage outputof the demodulator.

The output of theY demodulator is preferably passed through a iilter 48 designed to lter out second harmonic components of the signal voltage. Assuming that a 60 cycle signal voltage is employed, Iilter 48 is designed to lter out the 120 cycle voltage components. If desired, the outputof lter 48y may pass through an additional differentiating, or rate taking, circuit 49 which control; voltageY derived: from the. lten circuit.A

The output of the rate circuit 49 is then applied to a modulator 50 which may be of any suitable conventional type. The necessity of demodulating and then modulating occurs when the excitation voltage for the selsyn transmitters does not bear a xed phase relation with the supply voltage connected to the motor iield circuit.

In the embodiment of our invention. hereinl illustrated, the outputof modulator 50 is iiltered and then, applied toa novel circuitor phase-sen.- sitive output stage indicated generally at |00 which permits application of the signal andl bias voltages in parallel to the grids of the thyratron tubes 5|, 52, which are connected to control the direction and rate of operation of the servomotorindicated generally at 53 in accordance with the phase sense and magnitude of the signal voltage.

In the preferred embodiment of this circuit as shown in Fig. 2, the nixed bias for the tubes 5|- and 52 is obtained from a transformer 53|, the primary of which is connected across oneof the phases of the three-phase A. C. supply 54. The secondary of transformer 53| hasits center tap connected to ground and the opposite ends thereof are connected through condensers 55, 5B, respectively, and through resistors 51, 58, respectively, with the grids of thyratron tubes 5|, 52. These series condensers 55 and 56 are employed in order to permit parallel addition of the bias n and signal voltages onthe grids of the tubes.

The resistors 51 and 58 together with condensers 59and 60, which are connected to ground, function to prevent oscillations in the grid circuits.

The signal voltage derived from modulatorf is passed through transformer 6| and then supplied through coupling condenser 52 across a resonant lter circuit including inductance or the choke coil 63 and condenser 64 connected in series therewith. Assuming a 60 cycle control voltage as above assumed, the most objectionable harmonic frequency in the signal is the second harmonic or cycle voltage component. A filter of this type will serve to remove a maximum of the objectionable second harmonic with a minimum amount of phase delay to the 60 cycle control voltage. If a conventional 60 cycle tuned lter were to be employed or a low pass iilter, the delay occasioned thereby would nullify to some extent the rate eiect derived from the differentiating or rate taking circuits. Hence, with the resonant type of filter shown, a more stable con-v trol with less hunting is obtained.

The signal voltage after passing the series resonant lter circuit is applied in in-phase relation through resistors 55 and 66 to the grids of the thyratrons 5| and 52, being preferably connected as shown to points intermediate the condenser 55 and resistor 51 and condenser 56 and resistor 58. The plate potentials are applied, as pointed out in the following, in out-of-phase relation to the plates of the thyratron tubes 5| and 52. Also, the bias potentials derived through transformer 53 are applied in out-ofphase relation. As a result, clean firing of the thyratrons is achieved. Additionally, it will be noted that a path from each grid through resistors, rather than through any transformer-s secondaries or lter choices, exists to ground, thereby minimizing' or eliminating pulses in the gridv encarar:

7 squirrel cage motor, the armature of which drives load shaft 61, which in turn drives the rotors I4 and I5 of the signal transformers II and i2 through suitable gearing and shafting indicated generally as 68. The gear train B9 between the rotors I4 and I5' is such as to provide a 36:1 ratio between the' fine and coarse transformer rotors or' the same ratio as exists between the ne and coarse transmitters. The last-described connection between the servomotor output andV the signal transformers provides for Zeroing of the control signal when the load or positionable object driven by the servo is in synchronism or positional agreement with the reference member associated with the input shaft I.

' The field windings 'lil and II of the servomotor, as shown in Fig. 2, are connected across one phase of the three-phase supply 513 and in circuit with the-primaries 'l2 and 13 of transformers 14 and 75, field vwindings le and transformer winding l2 being in series thereacross, while field winding 1I and transformer winding 13 are in series thereacross. A condenser 'le is connected between a point intermediate one of the motor field windings and one of the transformer primary windings and another point intermediate the other motor field winding and the other transformer primary to provide a desired phase quadrature relation between the currents in the two phases of the field windings.

' The secondaries i8 and 19 of transformers 'lll and l5 are connected together at yone of the ends thereof and the other ends are connected respectively to the plates of the thyratrcn tubes I and 52. It will be observed that the transformers are so connected as to supply plate potentials to the tubes 5i and 52 in out-of-phase relationship as hereinabove indicated. rihe thyratron plate circuits serve to shunt the secondaries of transformers le and 'l5 to variable extents depending upon the conducting conditions of the tubes which, in turn, are dependent upon the magnitude of the signal voltages applied to the grids thereof and the phase sense of said signal voltages with respect to the plate potentials. In other words, a phase-sensitive amplifier results which controls the direction of rotation of the servomotor in accordance with the phase sense of the signal or signals derived from the coarse and une data transmission systems, and the rate of rotation will be primarily dependent upon the amplitude of these signal voltages.

If the two thyratron tubes nre equally producing equal currents in their plate outputs, the shunting effect of both tubes will be equal with the result that zero dierential currents will flow through the field windings of the two phase motor. However, when one of the plate currents predominates over the other, a differential current -will flow in the eld windings causingl the motor to rotate in one direction or the other depending upon which one of the tubes passes the predominating current. For a signal or control voltage of one phase sense, the phase relation of the current in field windings Til, ll will be of one sense, for example, that in winding 'l5 leading that in winding l I, and, upon a reversal of the phase sense of the control voltage, the phase relation of the current in these motor field windings will reverse or, in accordance with the above assumption, the current in winding 'II will lead that in winding l0.

While we have described our invention in its preferred embodiments, it is to be understood that the' Words which we have uesd are words of description ratherl than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of our invention in its broader aspects.

What is claimed is:

1. In a signal voltage mixing system of the character described, a circuit including 'a vpair of output terminals and two pairs of input terminals, a fine and coarse data transmission sys-7 tem including fine and coarse electrical signal transmitters and receivers connected respectively one with the other to provide fine and coarse alternating error signal voltages of like frequency to be mixed, the voltage outputs of said fine and coarsereceivers being connected respectively to the two pairs of input terminals, one of said pairs of input terminals being connected to receive the une signal voltage and being also connected with said output terminals to connect said ne voltage to said output terminals, and a pair of electron tubes each having a control electrode connected to receive the coarse alternating signal` voltage and their space discharge paths being connected in push-pull relation across said output terminals, said tubes being energized respectively to conduct on alternate half cycles of the signal voltage when said coarse signal is suiiicient to cause conduction such that both tubes will attenuate the fine signal when they conduct; whereby said tubes will provide a voltage across the output terminals proportional to said coarse signal voltage while controlling the magnitude of the fine signal voltage supplied to said output terminals so that the fine signal voltage across said output terminals progressively decreases as said coarse signal voltage increases and causes increased conductance of said tubes.

2. ln a signal voltage mixing system of the character described, a circuit including a pair of output terminals and two pairs of input terminals, a ne and coarse data transmission system including ne and coarse electrical signal transmitters and receivers connected respectively one with the other to provide fine and coarse alternating error signal voltages of like frequency to be mixed, the voltage outputs of said ne and coarse receivers being connected respectively to the two pairs of input terminals, one of said pairs of input terminals being connected to receive the fine signal voltage and with said output terminals to supply said fine voltage to said output terminals, and a pair of electron tubes each having a plate, cathode and control electrodes, a source of unidirectional plate voltage connected to said plates, the cathodes being connected together and the plates being connected respectively to the output terminals, and the pair of input terminals to which said coarse signal voltage is supplied being connected with said control electrodes to supply said coarse Signal in outof-phase relation thereto; whereby said tubes will provide a voltage across the output terminals proportional to said coarse signal voltage while controlling the magnitude of the fine signal voltage supplied to said output terminals so that the fine signal voltage across said output terminals progressively decreases as said coarse signal voltage increasesr and causes increased conductance of said tubes.

3. A system of the character recited in claim l further including a bias supply connected with said electron tubes for .preventing conduction thereof for small values of coarse signal voltage.

` s 4'. A system of the character recitedV in claim 9 2 further including means for supplyinga bias potential to said electron tubes for rendering them unresponsive to small values of coarse signal voltage.

RAWLEY D. MCCOY. THADDEUS J. KUSTO.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number 10 Name Date Schott June 22, 1937 Garman Mar. 18, 1941 Barr et al July 1, 1941 White June 23, 1942 Artzt July 13, 1943 Godet Sept. 17, 1946 Agins Oct. 22, 1946 Lesnick Nov. 26, 1946 Conklin July 1, 1947 Cawen July 22, 1947 Isbister et al July 29, 1947 McCoy Oct. 28, 1947 Fisher Dec. 9, 1947 Krupick Feb. 10, 1948 Edwards Aug. 10, 1948 Newell et al Aug. 31, 1948 McCoy June 20, 1950 Eller July 24, 1951 

